The purpose of this study is to test the parameters of a current model and develop a new mathematical model to predict simultaneously the total number of tumor cells and antibody-forming cells in the body of an animal. An immediate objective is measuring the average rate at which MOPC 104E tumor cells release IgM and determining the total number of tumor cells in vivo. Rates of release of antibody by antibody-forming cells have been made in this laboratory and various parameters are currently under investigation. Our preliminary studies indicate the feasibility of the model. Additional studies show that the various assumptions and parameters used are essentially correct. In tumor-bearing mice we are testing the effect of immunization on the animal's ability to respond to antigens. We are also testing the effects of chemotherapeutic agents and determining whether the model can predict the tumor cell population within the animal's body. The application of this model to all areas of clinical immunology is foreseen. Such a model is applicable to various problems associated with oncogenesis. If one could determine total number of tumor cells at any interval of time by measuring simple parameters, then such studies could be made on all types of neoplasms. The stage or extent of neoplastic involvement could possibly be clinically determined in man. Treatment of functional tumors producing a specific product could be monitored with this model.